Cheaper gas in Dixon; why?

Since I travel frequently through Dixon, I find gasoline is much cheaper in Dixon than in Rock Falls or Sterling. Cheaper meaning 35 cents a gallon. On April 28, I filled my tank for $3.34 a gallon.

Typically I find that gasoline is more expensive close to the interstate, but not so in Dixon. Since I live on the east end of Rock Falls, it is a mere 15 minutes with low traffic, no traffic lights, construction and one stop sign between the station and me – win-win. Located across the street are numerous chain stores, i.e. grocery, restaurants, big box, etc.

It puzzles me as to why gasoline prices in our towns are generally higher than the national average; why?